The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode converters including, for instance, an asymmetrical power converter.
A conventional asymmetrical power converter includes two (2) power switches coupled to a control circuit, at least one input/output isolation transformer, a rectifier and a filter. The asymmetrical power converter generally operates as follows. The first and second power switches conduct current in a complimentary manner to convert an input DC voltage into an AC voltage to be impressed across the isolation transformer. The rectifier then rectifies the voltage from the isolation transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load.
The control circuit monitors the output voltage of the asymmetrical power converter and adjusts the duty cycle of the power switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load.
In conventional asymmetrical power converters, a voltage conversion ratio (i.e., a ratio of the output voltage to the input voltage) for a given duty cycle may be represented by a hyperbolic function. The voltage conversion ratio typically starts at zero at a 0% duty cycle, increases hyperbolically to a peak voltage conversion ratio of 0.5 at a 50% duty cycle and decreases to zero at a 100% duty cycle. The voltage conversion ratio is hyperbolic instead of linear (as in the full-bridge power converter) and the slope of the hyperbolic function decreases to about zero at duty cycles approximating 50%. The control circuit selects the duty cycle of the power switches and, therefore, the voltage conversion ratio, to produce a desired output voltage relative to the input voltage. Once the proper duty cycle is determined, however, small fluctuations in the input voltage or the load may result in related fluctuations in the output voltage. The control circuit should, therefore, adjust the duty cycle of the power switches to maintain the output voltage at a relatively constant level.
The hyperbolic relationship between the duty cycle and the voltage conversion ratio, however, poses a substantial problem. Since the slope of the hyperbolic function is about zero at duty cycles approximating 50%, incremental adjustments to the duty cycle may result in negligible changes in the voltage conversion ratio and, therefore, the output voltage. The control circuit is usually complex to account for the varying voltage conversion ratio with respect to duty cycle. Alternatively, if a simple control circuit is employed, the asymmetrical power converter may need to be operated at a duty cycle substantially less than 50%. While the relationship between the duty cycle and the voltage conversion ratio may be more linear at duty cycles substantially less than 50%, the voltage conversion ratio is also lower, thereby resulting in a lower output voltage for a given input voltage.
The lower voltage conversion ratio necessitates a higher input voltage, resulting in a higher reverse voltage stress on the rectifying diodes of the rectifier. Further, operating the asymmetrical power converter at duty cycles substantially less than 50% may result in a significant difference in conduction losses of the two power switches. To minimize the reverse voltage stress on the rectifying diodes, an asymmetrical power converter having a higher voltage conversion ratio is desired. Additionally, an asymmetrical power converter capable of operating at about a 50% duty cycle is desirable to more evenly distribute power dissipation between the two power switches.
U.S. Pat. No. 5,838,552, entitled "Asymmetrical Power Converter and Method of Operation Thereof," issued to Fraidlin, et al. on Nov. 17, 1998, describes an asymmetrical power converter employing two transformers. The aforementioned reference is incorporated herein by reference. Fraidlin discloses an asymmetrical power converter that employs multiple transformers having differing turns ratios to achieve a reduction in reverse voltage in a rectifier coupled to a secondary winding of each of the transformers. Multiple transformers, however, require a significant amount of board real-estate. Additionally, the transformers are employed to store energy during alternate switching cycles and may, therefore, be of significant size.
Accordingly, what is needed in the art is an asymmetrical power converter that, while reducing board real-estate, provides a well-regulated output, and, at the same time, more evenly distributes voltage stresses within the converter to thereby achieve a highly efficient and cost effective converter.